In this example, we’ll show you how to write the formula for an ionic compound with a multivalent...

In this example, we’ll show you how to write the formula for an ionic compound with a multivalent metal.

Ionic Compounds

With Multivalent Metals

Example 1

A multivalent metal is a metal which can form more than one ion. Each ion it forms has a different positive charge.

Multivalent Metal - A metal which can form more than one ion, each with a different charge

On the periodic table you’re given, multivalent metals are the ones which have more than one charge in the top right corner. They are shown in this diagram.

Multivalent Metals

Here, we’ve included the two horizontal rows at the bottom of the periodic table, as many of these are multivalent.

Multivalent Metals

In this example, we’re asked to write the formula for the compound copper (II) chloride.

+2 –2

CuCl2

Cl–

Write the formula for the compound copper(II) chloride.

We find copper on the periodic table. It is a metal and it can form an ion with either a positive 2 charge, or a positive 1 charge.


The element we use for chloride is chlorine. It’s ion, the chloride ion has a charge of negative 1.


The periodic table tells us that copper can form either a positive 2 ion or a positive 1 ion.

Cu2+ Cl–

+2 –2

CuCl2

Cl–


How do we know what the charge will be on the ion??

Cu2+ Cl–

+2 –2

CuCl2

Cl–


?

Names of compounds with multivalent metals always have a roman numeral in brackets right after the name of the metal.

Cu2+ Cl–

+2 –2

CuCl2

Cl–


?roman numeral-means the charge on the ion

The roman numeral simply means the charge on the ion that is used in the compound.

Cu2+ Cl–

+2 –2

CuCl2

Cl–


?roman numeral-means the charge on the ion

So a roman numeral of 2 tells us the copper ion that’s used has a charge of positive 2.

Cu2+ Cl–

+2 –2

CuCl2

Cl–


?

So the copper ion in this compound is Cu 2 plus

Cu2+ Cl–

+2 –2

CuCl2

Cl–


The chloride ion has only one possible charge, and that is negative 1.

Cu2+ Cl–

+2 –2

CuCl2

Cl–


If we have one copper II ion and one chloride ion

Cu2+ Cl–

+2 –1

CuCl2

Cl–


Total Positive Charge

Total Negative Charge

The positive and negative charges are NOT balanced. They don’t add up to zero

Cu2+ Cl–

+2 –1

CuCl2

Cl–




These DO NOT add up to zero

To make the charges balance, we add another chloride ion (click)

Cu2+ Cl–

+2 –2

CuCl2

Cl–



The total negative charge is now negative 2.

Cu2+ Cl–

+2 –2

CuCl2

Cl–




Now the positive and negative charges are balanced. They add up to zero

Cu2+ Cl–

+2 –2

CuCl2

Cl–




NOW these add up to zero

There is one copper ion for every two chloride ions,

Cu2+ Cl–

+2 –2Cl–


So the formula is CuCl2

Cu2+ Cl–

+2 –2

CuCl2

Cl–


We have now answered the question. The formula for copper (II) chloride is CuCl2.

Cu2+ Cl–

+2 –2

CuCl2

Cl–


Formula:

Notice that the final formula has no charges and no roman numerals in it. In simple ionic compounds, roman numerals are used only in names, not in formulas.

Cu2+ Cl–

+2 –2

CuCl2

Cl–


Formula:

No charges!

No roman numerals!

In this example, we’ll show you how to write the formula for an ionic compound with a multivalent...

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